How to: Fertilize trees via liquid injection

Rex Bastian, The Care of Trees

When it comes to fertilizing trees, you'll find no shortage of options and
decisions. You have choices of how and how much fertilizer to apply, what
its nutrient content should be and much more. Examining all these options,
for which plenty of differing opinions exist, could fill a book. So to
provide a brief yet useful step-by-step guide to tree fertilization, this
article will focus on the most common method: liquid injection.

Liquid injection involves placing fertilizer as a liquid suspension or
solution into the soil around the tree. One advantage of liquid injection
is that by applying fertilizer below the soil surface, you can make certain
that nutrients, such as phosphorus, are more available to tree roots by
placing them below the thick turf zone.

Most managers' choice
If you are like most grounds managers, once you've decided on performing
liquid injection, you will use a fertilizer that is primarily slow-release
but also includes some fast-release components. For a general rule of thumb
in determining how much nitrogen to apply, use the rate suggested by the
forthcoming American National Standards Institute (ANSI) A300 guidelines: 2
to 6 pounds of actual nitrogen per 1,000 square feet. The ANSI guidelines
also suggest injection spacing and depth levels: 18 to 36 inches apart and
4 to 12 inches deep.

Of course, it goes without saying that you'll also need the proper
equipment to perform liquid-fertilizer injections, which basically means a
truck with a pressurized pumping system.

Step1: Determine how much fertilizer to inject
Before you can begin the injection process, you must calibrate the probe.
That is, you must determine the amount of fertilizer to deliver per
injection point. To do so, you need to consider the following:
*Injection spacing
*Analysis of the fertilizer (specifically the amount of nitrogen)
*Mixing rate of raw fertilizer per 100 gallons (described on the fertilizer
container)
*Rate of nitrogen per 1,000 square feet
*Probe flow rate (how many gallons per minute flow through the probe).
You can determine the probe flow rate by observing how fast the probe fills
a 5-gallon bucket and dividing the number of minutes, or fraction thereof,
into 5.
Knowing this information, let's consider an example to get started. Imagine
you have the following factors:
*3-foot injection spacing
*26-3-13 fertilizer (26 percent nitrogen)
*Mixing rate of 20 pounds of fertilizer per 100 gallons
*Desired nitrogen rate of 2 pounds actual nitrogen per 1,000 square feet
*A 9- x 9-foot area to fertilize.

Now run the math: A 9- x 9-foot area is 81 square feet. With a 3-foot
spacing, the area will contain nine injection sites.

But you want to use only 2 pounds of nitrogen per 1,000 square feet. So to
apply 2 pounds per 1,000 square feet, you need 40 gallons of fertilizer.

Now that you understand how much fertilizer you need for 1,000 square feet,
you can determine how much you need for 81 square feet. Figure the ratios
again:

40 gallons / 1,000 square feet = X gallons / 81 square feet

To solve for the number of gallons per 81 square feet:

3,240 (40 x 81) / 1,000 = 3.25 gallons of fertilizer

Let's say it took the probe 90 seconds (1.5 minutes) to fill a 5-gallon bucket.

5 gallons / 1.5 minutes = 3.3 gallons per minute

If you need 3.25 gallons of fertilizer and the probe delivers 3.3 gallons
per minute, you need roughly 1 minute of flow divided between the nine
injection holes in the area to be fertilized. To determine how long each
injection should last per hole:

Thus, leave the probe in each injection hole for roughly 6.5 seconds to
provide 2 pounds of nitrogen per 1,000 square feet.

(Bear in mind that these are rough guidelines. Factors such as soil
density, the number of probes you are using and the pressure of the system
all affect these values. If these variables change, you may have to
re-calibrate the probe. Unlike injecting medication into humans, varying
the injection amounts by as much as 25 percent is okay.)

Step 2: Prepare the injection site
Before you begin the injections, you must assess and prepare the area
around the tree. Check the ground. Are there irrigation heads? If so, try
to determine where the irrigation lines are so you do not damage them.
Check for gas lines, too. If you are at a residence and you see a gas grill
without a propane cylinder, a gas line may be running underground from the
house to the grill, so you'll need to watch for it too.

Are flower beds planted near the tree? If so, you will need to both step
carefully and watch that your hose does not drag through the flowers.

Be aware of property lines as well. Unless you have obtained written
consent from a neighbor, you can only fertilize the area of the tree that
is on your clients' property. (In which case, you should fertilize at the
pre-determined rate, not at an increased rate to compensate for partial
fertilizing.)

Step 3: Establish the injection grid
Most applicators base the injection grid on an imaginary box on the ground
that encompasses the canopy edges. The injections will fall within the box
at your pre-determined injection spacing.

Because the grid is something you'll likely want to simply "eyeball," it is
helpful to find a visual reference point beyond the tree's dripline. To
choose one, stand at the edge of the canopy. Pick a point that looks like
it is at the edge of the canopy on the other side of your box. Then pick
another point on a side of a building, or perhaps across the street, that
is beyond the original point. By sighting both points as you move through
your grid, you will be able to systematically and thoroughly cover the area
to be fertilized. This technique is particularly helpful if the ground is
uneven or if the area is not square.

Step 4: Begin the injections
Now that you have oriented yourself in the grid box, you are ready to begin
the injections:
*Stand at your starting point at the edge of the canopy.
*Insert the probe into the soil.
*Inject the fertilizer for the time calibrated. (In the above example, that
would be roughly 6 to 7 seconds.)
*Remove the probe.
*Walk 3 feet (again based on the above example) to your next grid point,
keeping an eye on your visual references.
*Insert the probe into the soil and repeat the process.
*Continue until you have reached the other side of the canopy. Step over 3
feet to begin the next row.
*Insert the probe and repeat the process until you have covered the entire
grid area beneath the canopy.

Bear in mind that trees have many roots growing close to the trunk. So it
is best to stay a foot or two away from it so that you can avoid driving
the probe into the large roots close to the soil surface.

You don't always have to use a grid pattern to determine the injection
area. For example, if you need to fertilize a conifer or other tree under
whose canopy you cannot reach, the injection pattern should be circular,
using about three rings. Perform the first ring of injections by reaching
inside the canopy. The second ring should be at the edge of the canopy. The
third injection ring should be about a foot or two outside the canopy.

Step 5: Inspect your injections
Whether you use the box or circular pattern, uniformity is important. About
2 to 4 weeks after you have fertilized the tree, tufts of greener grass
should appear at each injection site. Your client will be able to see if
your injections varied from the grid. If they have, your work will have an
unprofessional look to it.

Be sure to return to the site after you have fertilized a tree. Look for
the tufts of grass to see whether you adhered to the grid. And observe the
tree. Is it doing well? Does it look better or worse than before you
fertilized it? As you observe, remember that fertilization is not a
cure-all. Many other factors-such as soil moisture, amount of organic
material, soil compaction, root damage and so on-may actually affect a tree
more than fertilization.

Tree fertilization using the liquid-injection method takes some practice.
Calibrating the probe may seem complicated at first, but it does eventually
become easier. Likewise, following a grid pattern by sight does become more
efficient over time. The benefits to the tree are well worth the learning
curve.

Dr. Rex Bastian is director of technical services for The Care of Trees
with headquarters in Wheeling, Ill., and offices serving metropolitan
Washington, D.C., and New York City.